thr3ads.net - Linux Virtualization - [RFC v4 4/5] virtio_ring: add event idx support in packed ring [May 2018]

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Jason Wang

2018-May-16 12:17 UTC

[RFC v4 4/5] virtio_ring: add event idx support in packed ring

On 2018?05?16? 16:37, Tiwei Bie wrote:> This commit introduces the event idx support in
> packed ring.
>
> Signed-off-by: Tiwei Bie <tiwei.bie at intel.com>
> ---
>   drivers/virtio/virtio_ring.c | 75 +++++++++++++++++++++++++++++++++---
>   1 file changed, 70 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index c6c5deb0e3ae..de3839f3621a 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1006,7 +1006,7 @@ static inline int virtqueue_add_packed(struct
virtqueue *_vq,
>   static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	u16 flags;
> +	u16 new, old, off_wrap, flags, wrap_counter, event_idx;
>   	bool needs_kick;
>   	u32 snapshot;
>   
> @@ -1015,9 +1015,19 @@ static bool virtqueue_kick_prepare_packed(struct
virtqueue *_vq)
>   	 * suppressions. */
>   	virtio_mb(vq->weak_barriers);
>   
> +	old = vq->next_avail_idx - vq->num_added;
> +	new = vq->next_avail_idx;
> +	vq->num_added = 0;
> +
>   	snapshot = *(u32 *)vq->vring_packed.device;
> +	off_wrap = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot &
0xffff));
>   	flags = virtio16_to_cpu(_vq->vdev, (__virtio16)(snapshot >>
16)) & 0x3;
>   
> +	wrap_counter = off_wrap >> 15;
> +	event_idx = off_wrap & ~(1<<15);
> +	if (wrap_counter != vq->wrap_counter)
> +		event_idx -= vq->vring_packed.num;
> +
>   #ifdef DEBUG
>   	if (vq->last_add_time_valid) {
>   		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> @@ -1026,7 +1036,10 @@ static bool virtqueue_kick_prepare_packed(struct
virtqueue *_vq)
>   	vq->last_add_time_valid = false;
>   #endif
>   
> -	needs_kick = (flags != VRING_EVENT_F_DISABLE);
> +	if (flags == VRING_EVENT_F_DESC)
> +		needs_kick = vring_need_event(event_idx, new, old);
> +	else
> +		needs_kick = (flags != VRING_EVENT_F_DISABLE);
>   	END_USE(vq);
>   	return needs_kick;
>   }
> @@ -1098,7 +1111,7 @@ static void *virtqueue_get_buf_ctx_packed(struct
virtqueue *_vq,
>   					  void **ctx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> -	u16 last_used, id;
> +	u16 wrap_counter, last_used, id;
>   	void *ret;
>   
>   	START_USE(vq);
> @@ -1138,6 +1151,19 @@ static void *virtqueue_get_buf_ctx_packed(struct
virtqueue *_vq,
>   	ret = vq->desc_state[id].data;
>   	detach_buf_packed(vq, last_used, id, ctx);
>   
> +	wrap_counter = vq->wrap_counter;
> +	if (vq->last_used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	/* If we expect an interrupt for the next entry, tell host
> +	 * by writing event index and flush out the write before
> +	 * the read in the next get_buf call. */
> +	if (vq->event_flags_shadow == VRING_EVENT_F_DESC)
> +		virtio_store_mb(vq->weak_barriers,
> +				&vq->vring_packed.driver->off_wrap,
> +				cpu_to_virtio16(_vq->vdev, vq->last_used_idx |
> +						(wrap_counter << 15)));
> +
>   #ifdef DEBUG
>   	vq->last_add_time_valid = false;
>   #endif
> @@ -1160,15 +1186,27 @@ static void virtqueue_disable_cb_packed(struct
virtqueue *_vq)
>   static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 wrap_counter;
>   
>   	START_USE(vq);
>   
>   	/* We optimistically turn back on interrupts, then check if there was
>   	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always update the event index to keep code simple. */
> +
> +	wrap_counter = vq->wrap_counter;
> +	if (vq->last_used_idx > vq->next_avail_idx)
Should this be ">=" consider rx refill may try to completely fill
the ring?
> +		wrap_counter ^= 1;
> +
> +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> +			vq->last_used_idx | (wrap_counter << 15));
>   
>   	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
>   		virtio_wmb(vq->weak_barriers);
> -		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> +						     VRING_EVENT_F_ENABLE;
>   		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
>   							vq->event_flags_shadow);
>   	}
> @@ -1194,15 +1232,40 @@ static bool virtqueue_poll_packed(struct virtqueue
*_vq, unsigned last_used_idx)
>   static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
> +	u16 bufs, used_idx, wrap_counter;
>   
>   	START_USE(vq);
>   
>   	/* We optimistically turn back on interrupts, then check if there was
>   	 * more to do. */
> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> +	 * either clear the flags bit or point the event index at the next
> +	 * entry. Always update the event index to keep code simple. */
> +
> +	/* TODO: tune this threshold */
> +	if (vq->next_avail_idx < vq->last_used_idx)
> +		bufs = (vq->vring_packed.num + vq->next_avail_idx -
> +				vq->last_used_idx) * 3 / 4;
> +	else
> +		bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
> +
> +	wrap_counter = vq->wrap_counter;
> +	if (vq->last_used_idx > vq->next_avail_idx)
> +		wrap_counter ^= 1;
> +
> +	used_idx = vq->last_used_idx + bufs;
> +	if (used_idx >= vq->vring_packed.num) {
> +		used_idx -= vq->vring_packed.num;
> +		wrap_counter ^= 1;
> +	}
Looks correct but maybe it's better to add some comments for such logic.
> +
> +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev,
> +			used_idx | (wrap_counter << 15));
>   
>   	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
>   		virtio_wmb(vq->weak_barriers);
> -		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> +						     VRING_EVENT_F_ENABLE;
>   		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
>   							vq->event_flags_shadow);
>   	}
> @@ -1869,8 +1932,10 @@ void vring_transport_features(struct virtio_device
*vdev)
>   		switch (i) {
>   		case VIRTIO_RING_F_INDIRECT_DESC:
>   			break;
> +#if 0
>   		case VIRTIO_RING_F_EVENT_IDX:
>   			break;
> +#endif
Maybe it's time to remove this #if 0.

Thanks
>   		case VIRTIO_F_VERSION_1:
>   			break;
>   		case VIRTIO_F_IOMMU_PLATFORM:

Tiwei Bie

2018-May-16 12:58 UTC

head link

[RFC v4 4/5] virtio_ring: add event idx support in packed ring

On Wed, May 16, 2018 at 08:17:21PM +0800, Jason Wang
wrote:> On 2018?05?16? 16:37, Tiwei Bie wrote:
[...]> > @@ -1160,15 +1186,27 @@ static void virtqueue_disable_cb_packed(struct
virtqueue *_vq)
> >   static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue
*_vq)
> >   {
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	u16 wrap_counter;
> >   	START_USE(vq);
> >   	/* We optimistically turn back on interrupts, then check if there
was
> >   	 * more to do. */
> > +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> > +	 * either clear the flags bit or point the event index at the next
> > +	 * entry. Always update the event index to keep code simple. */
> > +
> > +	wrap_counter = vq->wrap_counter;
> > +	if (vq->last_used_idx > vq->next_avail_idx)
> 
> Should this be ">=" consider rx refill may try to completely
fill the ring?
It seems that there are two cases that last_used_idx
equals to next_avail_idx. The first one is that the
ring is empty. And the second one is that the ring
is full. Although in the first case, most probably,
the driver won't enable the interrupt.

Maybe I really should track the used_wrap_counter
instead of calculating it each time I need it.. I'll
give it a try..
> 
> > +		wrap_counter ^= 1;
> > +
> > +	vq->vring_packed.driver->off_wrap =
cpu_to_virtio16(_vq->vdev,
> > +			vq->last_used_idx | (wrap_counter << 15));
> >   	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> >   		virtio_wmb(vq->weak_barriers);
> > -		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> > +						     VRING_EVENT_F_ENABLE;
> >   		vq->vring_packed.driver->flags =
cpu_to_virtio16(_vq->vdev,
> >   							vq->event_flags_shadow);
> >   	}
> > @@ -1194,15 +1232,40 @@ static bool virtqueue_poll_packed(struct
virtqueue *_vq, unsigned last_used_idx)
> >   static bool virtqueue_enable_cb_delayed_packed(struct virtqueue
*_vq)
> >   {
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	u16 bufs, used_idx, wrap_counter;
> >   	START_USE(vq);
> >   	/* We optimistically turn back on interrupts, then check if there
was
> >   	 * more to do. */
> > +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> > +	 * either clear the flags bit or point the event index at the next
> > +	 * entry. Always update the event index to keep code simple. */
> > +
> > +	/* TODO: tune this threshold */
> > +	if (vq->next_avail_idx < vq->last_used_idx)
> > +		bufs = (vq->vring_packed.num + vq->next_avail_idx -
> > +				vq->last_used_idx) * 3 / 4;
> > +	else
> > +		bufs = (vq->next_avail_idx - vq->last_used_idx) * 3 / 4;
> > +
> > +	wrap_counter = vq->wrap_counter;
> > +	if (vq->last_used_idx > vq->next_avail_idx)
> > +		wrap_counter ^= 1;
> > +
> > +	used_idx = vq->last_used_idx + bufs;
> > +	if (used_idx >= vq->vring_packed.num) {
> > +		used_idx -= vq->vring_packed.num;
> > +		wrap_counter ^= 1;
> > +	}
> 
> Looks correct but maybe it's better to add some comments for such
logic.
Make sense.
> 
> > +
> > +	vq->vring_packed.driver->off_wrap =
cpu_to_virtio16(_vq->vdev,
> > +			used_idx | (wrap_counter << 15));
> >   	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> >   		virtio_wmb(vq->weak_barriers);
> > -		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC :
> > +						     VRING_EVENT_F_ENABLE;
> >   		vq->vring_packed.driver->flags =
cpu_to_virtio16(_vq->vdev,
> >   							vq->event_flags_shadow);
> >   	}
> > @@ -1869,8 +1932,10 @@ void vring_transport_features(struct
virtio_device *vdev)
> >   		switch (i) {
> >   		case VIRTIO_RING_F_INDIRECT_DESC:
> >   			break;
> > +#if 0
> >   		case VIRTIO_RING_F_EVENT_IDX:
> >   			break;
> > +#endif
> 
> Maybe it's time to remove this #if 0.
Will do it.

Thanks for the review!

Best regards,
Tiwei Bie

Jason Wang

2018-May-16 13:31 UTC

head link

[RFC v4 4/5] virtio_ring: add event idx support in packed ring

On 2018?05?16? 20:58, Tiwei Bie wrote:> On Wed, May 16, 2018 at 08:17:21PM +0800, Jason Wang wrote:
>> On 2018?05?16? 16:37, Tiwei Bie wrote:
> [...]
>>> @@ -1160,15 +1186,27 @@ static void
virtqueue_disable_cb_packed(struct virtqueue *_vq)
>>>    static unsigned virtqueue_enable_cb_prepare_packed(struct
virtqueue *_vq)
>>>    {
>>>    	struct vring_virtqueue *vq = to_vvq(_vq);
>>> +	u16 wrap_counter;
>>>    	START_USE(vq);
>>>    	/* We optimistically turn back on interrupts, then check if
there was
>>>    	 * more to do. */
>>> +	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need
to
>>> +	 * either clear the flags bit or point the event index at the
next
>>> +	 * entry. Always update the event index to keep code simple. */
>>> +
>>> +	wrap_counter = vq->wrap_counter;
>>> +	if (vq->last_used_idx > vq->next_avail_idx)
>> Should this be ">=" consider rx refill may try to
completely fill the ring?
> It seems that there are two cases that last_used_idx
> equals to next_avail_idx. The first one is that the
> ring is empty. And the second one is that the ring
> is full. Although in the first case, most probably,
> the driver won't enable the interrupt.
>
> Maybe I really should track the used_wrap_counter
> instead of calculating it each time I need it.. I'll
> give it a try..
>
Right, good to know and this will match spec sample code.

Thanks

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